1. Field of the Invention
The present invention relates to a photomask mainly used for projection aligners, a method for fabricating a photomask, and a method for fabricating a semiconductor device. More particularly, the invention relates to a photomask having a function of shifting a phase of projected exposure light passing through the photomask, a method for fabricating the photomask, and a method for fabricating a semiconductor device using such a photomask.
2. Description of the Related Art
Recently, in the fabrication of large-scale integrated circuits (LSIs), formation of very fine patterns are required. Consequently, in the exposure process in which fine circuit patterns are transferred onto semiconductor substrates (hereinafter referred to as “wafers”), photomasks having a function of shifting phases of exposure light to increase contrast (hereinafter referred to as “phase shift masks”) have been used.
Among them, an attenuated phase shift mask having a transmittance of several percent is widely used in the current manufacture of devices. This type of mask also has the function of providing a phase shift to increase contrast so that high-resolution pattern transfer is enabled. Hence this type of mask is preferred because of ease of fabrication, etc.
Generally in these phase shift masks, a phase shift pattern which shifts a phase of exposure light and a light-shielding pattern which blocks exposure light are disposed on a transparent substrate composed of quartz glass or the like.
The light-shielding pattern is provided in a region (hereinafter referred to as a “peripheral region”) in the periphery of a region for transferring a circuit pattern to a chip in the wafer (hereinafter referred to as a “main region”), and has a function of blocking unnecessary exposure light.
A conventional phase shift mask will be described below with reference to the drawings. FIG. 1A is a plan view schematically showing an example of a typical attenuated phase shift mask, and FIG. 1B is a cross-sectional view taken along the line I-I′ of FIG. 1A.
In a conventional attenuated phase shift mask 100, as shown in FIG. 1B, a phase shift pattern 102P composed of a translucent attenuated phase shift layer 102 and a light-shielding pattern 103P composed of a light-shielding layer 103 are deposited in that order on a transparent substrate 101.
In a main region 110, a pattern 140 corresponding to a circuit pattern provided on a wafer (hereinafter referred to as a “transfer pattern”) is disposed. This transfer pattern 140 is composed of the phase shift layer 102. Utilizing this transfer pattern 140, exposure light is transmitted with the phase of the exposure light being inverted by 180 degrees. Conversely, in the remaining regions of the transparent substrate 101 exposure light is transmitted with out inversion.
Note that projection exposure light is applied to the phase shift mask 100 in a direction indicated by the arrow L in FIG. 1B.
In a peripheral region 120, a light-shielding zone 130 is formed along the border between the main region 110 and the peripheral region 120. This light-shielding zone 130 prevents multiple patterning exposures to the adjacent chips during the transfer of the transfer pattern 140 to the wafer.
In the peripheral region 120, in addition to the light-shielding zone 130, various patterns are formed, such as alignment marks for aligning the aligner and the mask (fiducial patterns) 150. Furthermore, although not shown in the drawing, other patterns maybe formed in the peripheral region 120 as necessary. Such additional patterns may include, for example, a monitor pattern for measuring positional accuracy of the transfer pattern 140, a target pattern for alignment during each overlay-writing process, a pattern for measuring alignment accuracy used during each overlay-writing process, a pattern for alignment used in defect inspection of the transfer pattern 140, a pattern for alignment used during measurement of line width of the transfer pattern 140, a bar code pattern for identifying a mask, and a numbering pattern for identifying a mask.
A method for fabricating a conventional phase shift mask will now be described. FIGS. 2A to 2E and FIGS. 3F to 3I are each a cross-sectional view showing steps in a method for fabricating the conventional phase shift mask.
First, a phase shift layer 102 and a light-shielding layer 103 are deposited in that order on a transparent substrate 101, and a first resist layer 104 is then formed thereon (refer to FIG. 2A). As the first resist layer 104, a high-precision negative photoresist is used in order to improve patterning accuracy of a transfer pattern 140.
Subsequently, the first resist layer 104 is exposed and developed to form a first resist pattern 104P in the main region 110 and the peripheral region 120 (refer to FIG. 2B).
Using the resist pattern 104P as a mask, the light-shielding layer 103 and the phase shift layer 102 are sequentially etched. Thus, a light-shielding pattern 103P and a phase shift pattern 102P corresponding to the resist pattern 104P are formed (refer to FIGS. 2C and 2D).
The resist pattern 104P is removed (refer to FIG. 2E). Then, a second resist layer 105 of positive type is formed (refer to FIG. 3F). Exposure and development are then performed on the main region 110 to form a second resist pattern 105 having an opening corresponding only to the main region 110 (refer to FIG. 3G).
With respect to the pattern 140 exposed in the main region 110, portions composed of the light-shielding layer 103 are selectively etched so that only portions composed of the phase shift layer 102 remain in the main region 110 (refer to FIG. 3H). Finally, the second resist pattern 105 is removed to complete a phase shift mask 100 (refer to FIG. 3I).
As described above, in the conventional phase shift mask, by providing the light-shielding zone 130 in the peripheral region 120, unwanted exposure light does not enter the main region. As a result, adverse affects which maybe caused by the exposure light during the transfer of the circuit pattern to the wafer are prevented.
Furthermore, a blind mechanism (not shown) is provided on the aligner to block unnecessary exposure light. Such unnecessary exposure light is generally referred to as “stray light (flare)”.
Even if countermeasures for blocking stray light (flare) are taken as described above, stray light is not completely eliminated. Stray light is caused by reflection of exposure light from an illumination system and lenses of an aligner or a phase shift mask or the like. Hence, stray light still adversely affects the transfer of a circuit pattern to a wafer. As the patterns formed on wafers become finer, the effect of such stray light increases.
In order to overcome the problems associated with stray light, it is conceivable to cover the peripheral region 120 entirely with a light-shielding layer so that light is completely blocked. An example of such a structure is described, for example, in Japanese Unexamined Patent Application Publication No. 8-334885 (paragraphs [0017] and [0018], FIGS. 1 and 2.)
As the pattern formed on a semiconductor chip becomes finer, manufacturing cost of the photomask for forming the pattern increases. It is important to reduce manufacturing cost by decreasing exposure time when a pattern is formed in the photomask. Thus, it is necessary to decrease the area to be exposed by photolithography during the formation of the pattern.
From the standpoint of reduction in manufacturing cost, when the area remaining as a pattern by photolithography is small relative to the whole area (the whole area required for pattern formation on a surface of the photomask), a negative photoresist is used. This is because the negative photoresist allows only the portion exposed to remain (type (A)). In contrast, when the area remaining as a pattern is large relative to the whole area, a positive photoresist is used, because the positive photoresist allows only the portion exposed to be removed (type (B)).
In the above disclosed related art, since the phase shift mask disclosed in JP Pat. App. No. 8-334885 corresponds to type (B), a positive photoresist is assumed to be used (as the first resist layer 14 described in FIG. 3 of JP Pat. App. No. 8-334885).
On the other hand, the present inventor has observed that in view of patterning accuracy, the negative photoresist generally enables higher-precision patterning compared with the positive photoresist. Therefore, in the phase shift mask described in JP Pat. App. No. 8-334885, with respect to the formation of a transfer pattern, high patterning accuracy cannot be expected.
The inventor has observed that when a transfer pattern composed of a phase shift layer, such as a photomask for forming a gate, is formed by electron beam irradiation and a negative photomask is used, a portion irradiated with the electron beam becomes the transfer pattern after etching. Consequently, the accuracy of line width in the transfer pattern depends only on the energy profile of the electron beam applied.
The inventor has further observed that when a positive photomask is used, a portion irradiated with the electron beam corresponds to a pattern in which a transparent substrate is exposed (without formation of a phase shift layer). That is, the transfer pattern is formed by irradiating both sides of the transfer pattern with an electron beam. Consequently, the accuracy of line width in such a transfer pattern is greatly affected by the positional accuracy of irradiation of the electron beam in addition to the energy profile of the electron beam.
In a general photomask, such as a photomask for forming a gate, a negative photomask is more advantageous. On the other hand, in a photomask patterning wherein the exposed transparent substrate corresponds to a transfer pattern, as in the case for forming a hole layer, a positive photomask is more advantageous.
Additionally, when patterning is performed by type (A) photolithography, described above (i.e. if a negative photoresist with high patterning accuracy is used), a long period of time is required for exposure. This extended time results in an increase in manufacturing cost. Also, strain tends to occur in the photomask due to prolonged heat exposure.
As described above in the related art, in the phase shift mask disclosed in JP Pat. App. No. 8-334885, the light-shielding layer remains in the entire peripheral region. In addition, as discussed above, JP Pat. App. No. 8-334885 is type (B). Therefore, the related art does not achieve both reduction in manufacturing cost and high-precision patterning.
Additionally, the phase shift mask formation process is described in the JP Pat. App. No. 8-334885 disclosed in Patent Document 2 ((a)˜(e) of FIG. 3).